After a growing plant exhausts the nutrient resources stored in its seed, it begins to draw in nutrients from the surrounding soil using its root system. Rapidly growing plants have a high need for nutrients. If a plant cannot access the necessary nutrients then its growth becomes limited. Such nutrient limitation can impact the overall growth of the plant, and the economic return to the farmer. Farmers use a range of strategies for increasing the availability of nutrients for a growing crop, most notably the addition of chemical fertilizers, for example nitrogen and phosphorus. However, such chemical fertilizers can be lost from the field before providing any beneficial effect.
For example, nitrogen, which is commonly introduced to a field in the form of anhydrous ammonia or urea, can be lost through gas emission to the atmosphere or through run off as water drains from the field. In particular, ammonium, which is a positively charged ion, generally binds to soil particles and is resistant to loss via runoff. However, in alkaline conditions, ammonium transforms into its gaseous form, ammonia, which can be readily lost to the atmosphere. Ammonium can also be transformed into nitrate—and subsequently lost from the field—via a microbial process known as nitrification. Nitrate, on the other hand is a negatively charged ion and dissolves readily in water and can be lost as water runs off fields into drainage ditches or streams, or as water seeps downward into groundwater.
Nitrogen fertilizer containing urea is also susceptible to loss when applied to the soil surface. Specifically, when the urea is hydrolyzed, or broken down, it releases ammonia gas, which can be readily lost to the atmosphere. However, if the urea is hydrolyzed beneath the surface within the soil profile, there is a reduced chance that the ammonia gas will be lost.
Nitrogen from the various forms of fertilizer can also be lost through a process known as denitrification, whereby nitrate is converted to gaseous forms of nitrogen, including dinitrogen and nitrous oxide. And, nitrogen can also be lost through microbial-mediated processes that create other gaseous forms of nitrogen. Warmer soil temperatures cause microbial processes to occur more rapidly, meaning that nitrogen fertilizer remaining in or on warmer soils is increasingly susceptible to this type of loss.
Phosphorus, most commonly introduced to a field in the form of phosphate, generally has a lower loss rate than nitrogen, as phosphates readily bind to soil particles. Nevertheless, phosphorus can be lost from fields through soil erosion or, less commonly, via runoff if the soil can no longer bind additional phosphate because all of the available binding sites are filled.
Fertilizer costs, which are closely tied with the cost of fossil fuels, are significant in the production of commodity crops. Fertilizer that is lost from a field represents inefficiency in agricultural production systems, as well as a potential loss in profit realized by the farmer. The substantial cost of fertilizer in the production of commodity crops incentivizes farmers to adjust the application of fertilizer to closely match the needs of what they anticipate their crop will ultimately require throughout the growing season. Yet, because fertilizer is critical in boosting production, farmers are prone to over application out of anxiety that there will be insufficient nutrients available when they are required.
Particularly in the case of nitrogen fertilizer, the longer an externally-applied fertilizer remains on an agricultural field, the more opportunities there are for the fertilizer to be lost. Thus, ideally fertilizer is applied as needed throughout the growing season. However, tractor-drawn equipment generally cannot be used throughout the entire season. For example, corn plants, require nitrogen at least until reaching the point when tassels appear, which may be at a height of six feet or more. Conventional tractor-drawn implements are incapable of applying fertilizer when corn is so tall. This has led to the use of self-propelled sprayer systems, often referred to as “high boy” or “high-clearance” systems, capable of straddling tall crops. Airplanes commonly referred to as “crop dusters,” have been used to apply fertilizer throughout the growth season. But, unlike conventional tractor-drawn implements, high boy systems and crop dusters typically indiscriminately apply the fertilizer to the surface of the field.
Additionally, many farmers forego in season application, in favor of spring or fall applications, because of their anxiety about being able to get the equipment necessary to apply the fertilizer on the field within the appropriate time window for weather reasons. Farmers also contend with a range of tradeoffs when considering the timing of fertilizer applications, for example, the cost of fertilizer is often reduced in the fall as the demand for fertilizer diminishes. As a result, preseason applications of fertilizer—either in the late fall following harvest or around the time of planting in the spring—are common. Nevertheless, both fall and spring applied fertilizer has the potential of being lost from the field due to the various processes outlined above.
Inefficient use of fertilizer often also occurs when fertilizer is uniformly applied across an entire field. Many agricultural fields are heterogeneous, with one location potentially varying year-to-year in its nutrient status and differing from locations in other parts of the field. As a result, many farmers assess soil nutrient status with periodic samples analyzed in a laboratory. These soil tests are used to estimate nutrient needs prior to the growing season, in season, or prior to an in season application of fertilizer. Because of the effort required to take these samples, they are generally infrequent and representative of a rather large area on a given field. Thus, in addition to applying fertilizer in-season when nutrients are needed, an ideal application would also take into account the specific soil conditions locally within the field.
Besides optimizing the application of fertilizer by applying it in-season as nutrients are required, and tailoring the amount to suit the localized nutrient deficiencies of the soil within a field, the planting of cover crops can help reduce nutrient loss. Cover crops are generally grown on a field between the times when a commodity crop is grown. As cover crops grow, they take up and store nutrients, essentially preventing them from being lost from the field in runoff or in other ways. Some cover crops can absorb nitrogen from the atmosphere, and can augment the amount of soil nitrogen in a field, thereby reducing the need for future applications of fertilizer. Additionally, the roots of cover crops can reduce soil compaction and reduce soil erosion. Because some time is needed for germination, the ideal time to seed a cover crop on a corn field is after maturity when the corn plants are tall and their leaves are beginning to senesce or turn brown. Seeding at this time allows sufficient light for cover crop growth to penetrate the leaf canopy, enabling substantial growth of the cover crop to occur before the onset of winter.
More recently, there has been an interest in the use of small robotic vehicles on farms. The notion of a tractor that could navigate autonomously first appeared in patent literature in the 1980s. For example, U.S. Pat. No. 4,482,960, entitled “Robotic Tractors,” discloses a microcomputer based method and apparatus for automatically guiding tractors and other full sized farm machinery for the purpose of planting, tending and harvesting crops. One study in 2006 concluded that the relatively high cost of navigation systems and the relatively small payloads possible with small autonomous vehicles would make it extremely difficult to be cost effective as compared to more conventional agricultural methods. Accordingly, many of the autonomous vehicles that have been developed are as large as conventional tractors.
Despite the difficulty in maintaining cost effectiveness, a limited number of smaller agricultural robots have been developed. For example, the Maruyama Mfg. Co has developed a small autonomous vehicle capable of navigating between rows of crops. This vehicle, however, is limited to operating within a greenhouse, and is not suited for the uneven terrain typical of an agricultural field. Another example is U.S. Pat. No. 4,612,996, entitled “Robotic Agricultural System with Tractor Supported on Tracks,” which discloses a robotic tractor that travels on rails forming a grid over a crop field. However, use of this system requires the installation of an elaborate and potentially expensive track system within the agricultural field. Moreover, neither system is designed to carry a large payload, nor is either system capable of making sharp turns to navigate through the rows of a planted field on the uneven terrain of an agricultural field without causing substantial damage to the crops.
Accordingly, what is needed in the industry is a device which can autonomously navigate between the planted rows on the uneven terrain of an agricultural field to accomplish in-season management tasks, such as selectively applying fertilizer or other agricultural chemicals, mapping, soil sampling, and seeding cover crops when commodity crops grow to a height where use of convention tractor-drawn equipment or high clearance machines are no longer feasible. Moreover, what is needed by the industry is a device which can carry a large payload, is narrow enough to fit through the planted rows of crops, and can make sharp turns to navigate through the rows of a planted field to prevent excessive damage to planted crops.